Add a task for uploading nginx site config and a generator for configuring template before uploadinging

add a task ```cap ningx:config``` to upload nginx site config
add a generator ```rails g capistrano:nginx_puma:config``` for customizing template before uploading 
update readme
@seuros, i prefer not to touch template_puma function, maybe you can update it to check if it exists in config/deploy/templates .....

thanks !